The invention relates to a new process for the production of symmetrical bicyclo[3.3.0]octanedione dicarboxylic acid diesters. Optically active 6a-carba-prostacyclin and especially some compounds derived from it have a therapeutic use as stable analogs of the natural prostacyclin (PGI.sub.2) [R. C. Nickolsen, M. H. Town, H. Vorbrueggen: Prostacyclin-Analogs, Medicinal Research Reviews, Vol. 5, No. 1, pp. 1-53 (1985)]. The syntheses specified in this recent survey are long and lead partly only to racemic carbacyclins. The syntheses, which lead to carbacyclins in the absolute configuration corresponding to the natural PGI.sub.2, are especially expensive. The reason for this is that easily accessible suitable initial materials are achiral and the optical activity must be introduced in the course of the synthesis only in intermediate stages suitable for this purpose.
It is known that symmetrical prochiral dicarboxylic acid diesters of prostacyclin and carbacyclin intermediate stages can be saponified and decarboxylated enantioselectively to the monocarboxylic acid esters in very good yields, if enzymes, especially alpha-chymotrypsin, are used for this purpose (DE 36 38 760.6). ##STR1##
Symmetrical diester I used as initial material for this process is produced, according to literature methods, from 4-acetoxy-2-cyclopenten-1-one and acetone dicarboxylic acid ester [V. Osterthun and E. Winterfeldt, Chem. Ber. 11, 146 (1977) M. Harre, P. Raddatz, R. Walenta and E. Winterfeld, Angew, Chem. 94, 496 (1982)]. In both publications the acetoxy group in the 4-acetoxy-2-cyclopenten-1-one is stressed as the leaving group necessary for the reaction. But the production of 4-acetoxy-2-cyclopenten-1-one proves to be especially expensive and, in addition, uneconomical. Up to now no economical synthesis has been found for this purpose and it has been necessary to resort to the expensive initial materials 4-bromo-2-cyclo-penten-1-one and silver racemate.